1. Field of the Invention
This invention relates to the field of photography, and in particular to a method and apparatus for sharpening camera-recorded pictures. In a first implementation of the invention, said method and apparatus relate to flattening the film against a reference surface such as the surface of rollfilm pressure-plate or a sheetfilm-holder partition substantially more rigorously than is the case in conventional cameras, and furthermore to a novel focusing screen for view, i.e. large-format, cameras replacing the conventional screen and its conventional accessories and allowing focusing with heretofore unavailable reliability. In a second implementation of the invention, said method and apparatus relate to improved focusing of view cameras.
It will be understood by those skilled in the art that the word "photography" is intended to encompass any application in which an optical image is captured by a light sensitive medium or "film."
2. Description of Related Art
One serious but much neglected problem encountered in photography is that warping of the film will cause the film to deviate from the image plane, and make it impossible to achieve optimal image sharpness. No matter how accurate the optical components of the image capture device or "camera," an ideally geometrically sharp image is, in practice, impossible to obtain using conventional equipment due to warpage of the film and consequently separation of the film from the pressure plate (or sheetfilm holder partition), conventionally provided as film-plane reference-surface means, will take place.
Once normal focusing of the film has been carried out in a camera only conventionally fitted with a pressure plate or sheetfilm-holder partition, the film warp is typically of sufficient magnitude to be the critical parameter in determining image sharpness. The problem is most acute for medium format films such as 120/220 films, but it also significantly affects 35 mm and large-film formats, and it affects all conventional cameras regardless of price.
Aside the intrinsic film warp, the 120 film format in particular is subject to further film detachment due to the sandwiched paper backing used in this format and which during picture taking will be sandwiched between the film support holding the emulsion at its top and the pressure plate or partition. Moreover, even if the paper backing were removed, and palliation achieved to that extent, the bare film still would excessively detach per se from the pressure plate and would still preclude uniform, sharp focusing.
Attempts have been made to address the problem of rollfilm or sheetfilm detachment by providing pneumatic suction devices that will pull the film against the film reference-surface means. Such suction devices, however, are inherently bulky, cumbersome and thereby unsuitable for use, both outdoors and indoors, except in exceedingly circumscribed conditions.
Other attempts to flatten the film in the camera have resorted to compressing the film between a glass plate and the pressure plate. Such configurations however entail the substantive drawbacks of inserting into the imaging path another optical element which, being in contact with the emulsion, moreover will inherently soil quickly. Such "solutions" have long been abandoned.
Consequently, aside from a few unsatisfactory attempts at remedy, the problem of film detachment has as a rule been ignored. Currently, there do not appear to be any significant efforts to address the problem, as evidenced for example by the fact that the text Image Clarity by John B. Williams, 224 pp, Focal Press, 1990, which deals solely with image sharpness, fails to address the problem of film detachment.
It is true that the state of the art already comprises photographic films with layers of magnetic material, and that one aspect of the present invention involves the use of a multilayer laminate affixed to a film (in cooperation with a force-field generating unit). However the mounting of a multilayer laminate on a photographic film as used in the present invention is clearly distinguishable from prior art proposals to provide photographic emulsions with layers of magnetic material spread across the emulsion. These magnetic layers of the films of the prior art are used for magnetic recording, for instance of sound, and would have been recognized by one of ordinary skill in the art as being unsuitable for use, in combination with a force-field generating apparatus, to flatten a photographic film in order to cause the image plane and the film to rigorously coincide and thereby improve the sharpness of the image: the magnetic material of the prior art is unsuitable for the purposes of the present invention because it is diaphanous and permanently in place, whereas the magnetic laminate of the invention is opaque and removed subsequent to exposure, and moreover the prior art arrangements degrade image quality both by light (i.e., the image) being scattered by the magnetic particles and by raising the fog level, whereas the present invention improves image quality without affecting fog levels. Publications disclosing such prior art combinations of film with magnetic layers include:
U.S. Pat. Nos. 2,604,831; 3,782,947; 3,993,488; 4,279,945; 4,341,855; 5,254,449; French patent 1,651,889; Canadian patent 686,172; Japanese patents 57-151,926; 4-214,544.
It may be worthwhile to observe that what may appear to be a potential alternative way to flatten a flexible film, i.e., increasing the tension on the film, while effective where much slack is present, is intrinsically and in practice unable to fully flatten the film and indeed must always fall very short of the goal of removing film warp as a limiting factor in achieving optimal image sharpness.
Considering the efforts and resources conventionally devoted to improving the quality of film and optical components, then by removing said film warp, the present invention shall provide significant benefits by making possible in economical and practical manner substantially higher image sharpness.
Another aspect of the invention relates to a novel focusing screen for "view", i.e. large-format cameras. Conventional view cameras typically comprise a lens connected in light-tight manner to a light-tight bellows itself connected in light tight-manner to a first rear frame that will receive the sheetfilm holder at least during picture taking. This first rear frame is fitted with a second rear frame, often called "back", which holds the focusing screen, usually with one ground glass surface or equivalent on which the lens image is reproduced and its sharpness used as a focusing gauge. This second frame can be pulled back enough from the first frame to allow inserting and holding the sheetfilm holder at least during image exposure.
This configuration assumes that the plane of the ground glass surface for which focusing was carried out also is the same plane occupied by the emulsion of the sheetfilm in the sheetfilm holder when inserted between said first and second frames, and moreover that once such a coincidence were or has been established, it would or shall hold into the future too. However to monitor such coincidence, special apparatus (autocollimators) are required, which are expensive, can be used practically only in a lab, in other words not by the photographer in the field.
Tests run by the inventor on a so-called 4.times.5 camera, the smallest of the view cameras, indicate that, for a "normal" or near-normal lens focussed at infinity, the discrepancy in distance between lens and film plane relative to perfect focus preferably shall be kept within 50 microns (1/20 mm) but that the coincidence of said emulsion plane and ground-glass surface is unlikely to be realized within such a narrow range in typical view cameras. This drawback is compounded by the image resolution of the ground-glass surface being too coarse to detect a focusing shift of some 50 microns.
The invention offers substantial palliations to remedy the just cited drawbacks of view cameras. It does so by bypassing the conventional focusing screen in said rear frame, this rear frame itself being also present in the invention to conventionally hold the conventional sheetfilm holder in place, and using instead of said conventional focusing screen an insertable focusing frame of the invention, outwardly similar to a conventional sheetfilm holder and also having a partition, however in the form of a viewing screen having a focusing surface, as the novel focusing screen. In this manner the photographer is equipped at all times with a highly reliable and very portable reference means allowing him to positively establish congruence between the focusing and the imaging planes without loss of time.
This first above basic design of the novel focusing screen of the invention can be further refined and such additional measures are elucidated further below.